Surface sensitivity of atomic resolution secondary electron imaging.

The surface sensitivity of high-resolution secondary electron (SE) imaging is examined using twisted bilayers of MoS2 stacked at an angle of 30-degree. High-resolution SE images of the twisted bilayer MoS2 show a honeycomb structure composed of Mo and S atoms, elucidating the monolayer structure of MoS2. Simultaneously captured annular dark-field scanning transmission electron microscope images from the same region show the projected structure of the two layers. That is, the SE images from the bilayer MoS2 selectively visualize the surface monolayer. It is noted that SE yields from the surface monolayer are approximately 3 times higher than those from the second monolayer, likely attributable to attenuation when SEs emitted from the second layer traverse the surface layer. Mini abstract: The surface sensitivity of atomic resolution secondary electron imaging is examined using MoS2 bilayers, the thinnest system composed of a surface layer and substrate. This study reveals that the secondary electrons visualize the atomic arrangement of the surface monolayer three times more intensely than that of the second layer.